Just wondering of something might be wrong that I should check into. Truck is super slow to warm up. I live in Georgia so it's not too cold. Once it does finally get warm to 180-190ish, just a min or 2 of idleing and the gauge is already going back down. Can't remember if this is normal. And I've been running 5w40 for a while.dont know if that matters

 

Sounds like a thermostat issue. Granted you may have replaced it within the last few months, but it was probably in a bad batch.

 

That's true it prob was. So think it might be creeping open or staying open huh

 

What temp does the gauge go back down to? does the temp fluctuate?
I had a problem like this with my 99 where the temp would fluctuate rapidly after it warmed up and that's the tell tale sign of a head gasket, and thats what it was for me.

 

Just back down to mid hundreds. When it's sitting idling. It's like it will only warm to normal if it's working hard. Interstate it will stay at 180 ish.

 

Well that doesn't sound like a head gasket symptom. only if you can visually see the needle moving slowly back and forth.
A thermostat isn't difficult to replace and is likely the issue. If you haven't changed it you should soon because its not good for your engine to run cold for long periods, bad wear can occur especially in the upper end. Oil viscosity has alot to do with it. I saw a powerstroke motor that ran for a long time with a bad thermostat stuck open so the engine wouldn't warm up, the rollers on the lifters had worn into the cam a good 1/8 inch due to the lack of hot oil to the top end.
Ive used 15-40 Rotella both summer and winter for years and have had 0 problems.
the oil viscosity changes as it warm up and motor oil does not work correctly when cold especially with diesels, I would recommend getting the hottest thermostat you can for your application, you may even notice a power increase.